Proton FLASH effects on mouse skin at different oxygen tensions

Abstract

Objective. Irradiation at FLASH dose rates (>40 Gy s^-1) has received great attention due to its reported normal tissue sparing effect. The FLASH effect was originally observed in electron irradiations but has since been shown to also occur with both photon and proton beams. Several mechanisms have been proposed to explain the tissue sparing at high dose rates, including effects involving oxygen, such as depletion of oxygen within the irradiated cells. In this study, we investigated the protective role of FLASH proton irradiation on the skin when varying the oxygen concentration.Approach. Our double scattering proton system provided a 1.2 × 1.6 cm²elliptical field at a dose rate of ∼130 Gy s^-1. The conventional dose rate was ∼0.4 Gy s^-1. The legs of the FVB/N mice were marked with two tattooed dots and fixed in a holder for exposure. To alter the skin oxygen concentration, the mice were breathing pure oxygen or had their legs tied to restrict blood flow. The distance between the two dots was measured to analyze skin contraction over time.Main results. FLASH irradiation mitigated skin contraction by 15% compared to conventional dose rate irradiation. The epidermis thickness and collagen deposition at 75 d following 25 to 30 Gy exposure suggested a long-term protective function in the skin from FLASH irradiation. Providing the mice with oxygen or reducing the skin oxygen concentration removed the dose-rate-dependent difference in response.Significance. FLASH proton irradiation decreased skin contraction, epidermis thickness and collagen deposition compared to standard dose rate irradiations. The observed oxygen-dependence of the FLASH effect is consistent with, but not conclusive of, fast oxygen depletion during the exposure.

Keywords: flash effect; oxygen; proton; skin.

Figure 1.

(a): The experimental setup for the FLASH proton irradiation of the mouse skin, (b): The ink spots on the skin of the rear leg.

Figure 2.

(a)-(c): The skin contraction curves post 25 Gy (a) and 27 Gy (b) irradiation with normal air inhalation as well as the combination analysis of 25 Gy, 27 Gy and 30 Gy groups (c). (d)-(e): The skin contraction curves post 27 Gy (d) and 30 Gy (e) irradiation with oxygen inhalation, (f): The combination analysis of the oxygen inhalation groups following 27 Gy and 30 Gy exposure. n= 10 mice/group, *P<0.05.

Figure 3.

(a): The trichrome stained skin tissue images post irradiation (75 days after 25 Gy and 27 Gy; 50 days after 30 Gy and 45 Gy) at various oxygen concentrations. The collagen was dyed blue, (b): The epidermis thickness of the skin post irradiation, (c): The collagen deposition of the skin post irradiation. n= 10 mice/group, *P<0.05, **P<0.01.